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Date: Wed, 5 Dec 2018 01:09:18 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "luto@...nel.org" <luto@...nel.org>, "nadav.amit@...il.com"
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	"Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

On Tue, 2018-12-04 at 14:48 -0800, Nadav Amit wrote:
> > On Dec 4, 2018, at 11:48 AM, Andy Lutomirski <luto@...nel.org> wrote:
> > 
> > On Tue, Dec 4, 2018 at 11:45 AM Nadav Amit <nadav.amit@...il.com> wrote:
> > > > On Dec 4, 2018, at 10:56 AM, Andy Lutomirski <luto@...nel.org> wrote:
> > > > 
> > > > On Mon, Dec 3, 2018 at 5:43 PM Nadav Amit <nadav.amit@...il.com> wrote:
> > > > > > On Nov 27, 2018, at 4:07 PM, Rick Edgecombe <
> > > > > > rick.p.edgecombe@...el.com> wrote:
> > > > > > 
> > > > > > Since vfree will lazily flush the TLB, but not lazily free the
> > > > > > underlying pages,
> > > > > > it often leaves stale TLB entries to freed pages that could get re-
> > > > > > used. This is
> > > > > > undesirable for cases where the memory being freed has special
> > > > > > permissions such
> > > > > > as executable.
> > > > > 
> > > > > So I am trying to finish my patch-set for preventing transient W+X
> > > > > mappings
> > > > > from taking space, by handling kprobes & ftrace that I missed (thanks
> > > > > again for
> > > > > pointing it out).
> > > > > 
> > > > > But all of the sudden, I don’t understand why we have the problem that
> > > > > this
> > > > > (your) patch-set deals with at all. We already change the mappings to
> > > > > make
> > > > > the memory writable before freeing the memory, so why can’t we make it
> > > > > non-executable at the same time? Actually, why do we make the module
> > > > > memory,
> > > > > including its data executable before freeing it???
> > > > 
> > > > All the code you're looking at is IMO a very awkward and possibly
> > > > incorrect of doing what's actually necessary: putting the direct map
> > > > the way it wants to be.
> > > > 
> > > > Can't we shove this entirely mess into vunmap?  Have a flag (as part
> > > > of vmalloc like in Rick's patch or as a flag passed to a vfree variant
> > > > directly) that makes the vunmap code that frees the underlying pages
> > > > also reset their permissions?
> > > > 
> > > > Right now, we muck with set_memory_rw() and set_memory_nx(), which
> > > > both have very awkward (and inconsistent with each other!) semantics
> > > > when called on vmalloc memory.  And they have their own flushes, which
> > > > is inefficient.  Maybe the right solution is for vunmap to remove the
> > > > vmap area PTEs, call into a function like set_memory_rw() that resets
> > > > the direct maps to their default permissions *without* flushing, and
> > > > then to do a single flush for everything.  Or, even better, to cause
> > > > the change_page_attr code to do the flush and also to flush the vmap
> > > > area all at once so that very small free operations can flush single
> > > > pages instead of flushing globally.
> > > 
> > > Thanks for the explanation. I read it just after I realized that indeed
> > > the
> > > whole purpose of this code is to get cpa_process_alias()
> > > update the corresponding direct mapping.
> > > 
> > > This thing (pageattr.c) indeed seems over-engineered and very unintuitive.
> > > Right now I have a list of patch-sets that I owe, so I don’t have the time
> > > to deal with it.
> > > 
> > > But, I still think that disable_ro_nx() should not call set_memory_x().
> > > IIUC, this breaks W+X of the direct-mapping which correspond with the
> > > module
> > > memory. Does it ever stop being W+X?? I’ll have another look.
> > 
> > Dunno.  I did once chase down a bug where some memory got freed while
> > it was still read-only, and the results were hilarious and hard to
> > debug, since the explosion happened long after the buggy code
> > finished.
> 
> This piece of code causes me pain and misery.
> 
> So, it turns out that the direct map is *not* changed if you just change
> the NX-bit. See change_page_attr_set_clr():
> 
>         /* No alias checking for _NX bit modifications */
>         checkalias = (pgprot_val(mask_set) | pgprot_val(mask_clr)) !=
> _PAGE_NX;
> 
> How many levels of abstraction are broken in the way? What would happen
> if somebody tries to change the NX-bit and some other bit in the PTE?
> Luckily, I don’t think someone does… at least for now.
> 
> So, again, I think the change I proposed makes sense. nios2 does not have
> set_memory_x() and it will not be affected.
> 
Hold on...so on architectures that don't have set_memory_ but do have something
like NX, wont the executable stale TLB continue to live to re-used pages, and so
it doesn't fix the problem this patch is trying to address generally? I see at
least a couple archs use vmalloc and have an exec bit, but don't define
set_memory_*.

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